1. Field of the Invention
The present invention relates to a method for controlling AC pulse arc welding, by which stable welding is performed even when an electrode negative polarity current ratio is high.
2. Description of the Related Art
AC pulse arc welding proceeds by repeating cycles each consisting of an electrode positive polarity period (in which a peak current and a base current are applied) and an electrode negative polarity period (in which a base current is applied). In AC pulse arc welding, it is possible to adjust the amount of heat inputted to the base metal by adjusting the electrode negative polarity period, thereby varying the electrode negative polarity current ratio. As a result, welding with low-heat input can be performed, which enables high-quality welding with respect to thin plates. The capability of varying the electrode negative polarity current ratio also makes it possible to optimize penetration depth, weld reinforcement height, and other bead shape factors, depending on the work. Hereinafter, a conventional method of AC pulse arc welding (see Japanese Laid-open Patent Publications No. 2002-86271 and 2007-283393, for example) will be described.
FIG. 9 is a waveform chart of typical electric current and voltage used in AC pulse arc welding. FIG. 9(A) shows a welding current Iw whereas FIG. 9(B) shows a welding voltage Vw. In FIGS. 9(A) and (B), the upper sides above 0 A and 0 V represent the electrode positive polarity (EP) state, while the lower sides below 0 A and 0 V represent the electrode negative polarity (EN) state. The welding wire is fed at a predetermined wire feeding rate. In switching the polarity from one to the other, a high voltage of a several hundred volts is applied across the welding wire and the base metal for a very short period of time in order to prevent the breakage of the arc.
As shown in FIGS. 9(A) and (B), during the electrode negative polarity period Ten, a predetermined base current Ibn flows, and a predetermined base voltage Vbn is applied. The electrode negative polarity base current Ibn is set to be 20-200 A, for example, which is lower than a critical value in order to avoid droplet formation at the tip of the welding wire. The critical value is defined as the value of a welding current at which the welding wire's droplet transfer state changes to a spray transfer state. The critical value depends upon, for example, the material of the welding wire and the kind of the shielding gas employed. For instance, the critical value may be about 350 A for aluminum wire (shielding gas: argon), which is often used in AC pulse arc welding. For steel wire (shielding gas: a mixture of argon gas 80% and carbon dioxide gas 20%), the critical value may be about 450.
The electrode positive polarity period Tep is divided into a peak period Tp and a base period Tb. During the peak period Tp, as shown in FIGS. 9(A) and (B), a predetermined peak current Ip, larger than the critical value, is caused to flow upon application of a peak voltage Vp, in order to achieve the transfer of a droplet. The peak period Tp and the peak current Ip are so adjusted as to attain so-called “1 pulse-1 droplet” transfer, that is, a single droplet is transferred to the molten pool upon a single application of peak current Ip in one cycle. This ensures stable welding. During the base period Tb, a predetermined base current Ib, smaller than the critical value, is caused to flow upon application of a base voltage Vb, so as not to form a droplet. The base current Ib may be 20-80 A, for example.
One pulse cycle Tf is made up of the above-described electrode negative polarity period Ten, peak period Tp and base period Tb, and the welding is performed by repeating the cycle Tf. The electrode negative polarity period Ten and the peak period Tp are predetermined period, whereas the base period Tb is a period determined by feedback control performed for optimizing the arc length. In the arc length control, the length of the base period Tb is controlled so that an average value Vav of the absolute value of the welding voltage Vw in FIG. 9(B) is equal to a predetermined voltage setting value.
The formation and transfer of a droplet in AC pulse arc welding will be summarized as follows. A droplet transfer occurs around the end of the peak period Tp (i.e. immediately before, exactly at, or immediately after the end of the peak period Tp). Then, during the subsequent period of base period Tb, a base current Ib which has a small current value lower than the critical value is applied, so that the tip of the welding wire hardly melts and no droplet is formed. During the subsequent electrode negative polarity period Ten, an electrode negative polarity base current Ibn, smaller than the critical value, is caused to flow. Though having an equally small value, the electric current has a greater capability of melting the welding wire tip in the electrode negative (EN) polarity state than in the electrode positive (EP) polarity state. However, the electrode negative polarity period Ten is a short period because AC pulse arc welding is typically performed with the electrode negative polarity current ratio lying in a range of 0 through 30%. Therefore, only a small part of the welding wire will melt, resulting in the formation of a very small droplet. During the subsequent period of peak period Tp, a peak current Ip, larger than the critical value, is caused to flow. Accordingly, the welding wire tip melts enough to form a substantially large droplet. At this stage, the applied peak current Ip generates an electromagnetic force or pinch that acts on an upper portion of the droplet, thereby producing a constricted part or a neck in the droplet. Then, around the end of the peak period Tp, the constricted part becomes much thinner, and finally the droplet is transferred to the molten pool. In DC pulse arc welding, the formation and transfer of a droplet also takes place during the peak period Tp. As described above, by achieving the 1 pulse-1 droplet transfer (i.e. one droplet transfer takes place each cycle), a stable welding state is produced, and high-quality welding will result.
The electrode negative polarity current ratio Ren(%) is defined as follows:Ren=((Ten·|Ibn|)/(Ten·|Ibn|+Tp·Ip+Tb·Ib))×100
As seen from this formula, the ratio Ren represents the proportion of welding current during the electrode negative polarity with respect to an average value of welding current absolute values.
In the mathematical expression given above, the peak current Ip and the base current Ib are predetermined values, and so is the peak period Tp. The base period Tb can be regarded as a predetermined constant under a normal state where the arc length has an appropriate value. Therefore, it is possible to adjust the ratio Ren by adjusting the electrode negative polarity period Ten and/or the electrode negative polarity base current Ibn. Depending on the ratio Ren, the states of the penetration and reinforcement will vary, resulting in changes in the bead shape.
As described above, in AC pulse arc welding, it is common to choose an appropriate value from a range of 0 through 30% for the electrode negative polarity current ratio, depending on the work. A 0% electrode negative polarity current ratio means DC pulse arc welding. The electrode negative polarity current ratio selected from the above range does not cause the droplet to grow too large in the electrode negative polarity period Ten, and it is therefore possible to achieve a droplet formation and transfer in the peak period Tp.
However, depending on the work, it is necessary to achieve smaller penetration and larger reinforcement, i.e. it is necessary to form a bead shape with a low dilution rate. An example of such a case is when high-speed welding is performed with respect to thin steel plates with a large gap present at the welding joint. In this case, a bead shape with a low dilution rate is required in order to fill the gap with the molten metal while attaining a small penetration. In order to form such a bead shape, the ratio Ren needs to be set to a value which is beyond the above-described normal range, i.e. over 30%, or even over 50%. Conventionally, the ratio Ren can be set to a high value by setting the electrode negative polarity period Ten and/or the electrode negative polarity base current Ibn to a large value. As a result of this, the welding wire tip melts during the electrode negative polarity period Ten, and a large droplet is formed. In this state, the welding process goes into the peak period Tp, and the droplet grows much larger during the peak period Tp. Such a large droplet, however, cannot be transferred completely, and some molten part will remain on the welding wire tip even at the end of the peak period Tp. This residual droplet affects the transfer of the next droplet, making it impossible to attain the 1 pulse-1 droplet transfer. In other words, the droplet transfer occurs at random, and the welding state becomes unstable.